This invention relates in general to an ion beam gun and an ion beam exposure device, and in particular to an ion beam exposure device utilizing a gas ion beam gun and a mask.
Ion beam guns making use of gas ionization are well known in the art. One such prior art device applies an electrical field to a tungsten pin which is wetted on its top surface by condensed hydrogen and emits an atomic or molecular ion. In a second prior art device an electrical field is applied to a metal film having a small pin hole. The electrical field is generated by applying hydrogen gas emitted at the pin hole to the metal layer.
A stencil mask has been used as an ion beam exposure mask during ion beam exposure of the prior art ion beam guns. The stensile mask is prepared by selectively etching an extremely thin film of tungsten, molybdenum or the like, thereby producing holes in the form of the desired pattern on the film. Another known mask includes a membrane of silicon, Al.sub.2 O.sub.3, polyimide and the like. The masks may also be a metal film which blocks the ion beam from passing disposed on one of the membranes in the form of desired patterns.
Reference is made to FIGS. 1 and 2 wherein a prior art ion beam 14 as disclosed in J. L. Bartelt, "Masks Ion Beam Lithography: The Following Generation New Technology", Solid State Technology, August 1986, pp. 44-51 is shown. FIG. 1 shows a channeling mask of a monocrystalline silicon substrate 10 having a silicon epitaxial layer 11 formed thereon. A plurality of grooves 15 are partially formed on one surface of substrate 10 to provide regions of relatively lesser thickness forming a pattern. Grooves 15 are of different depths and widths to form a variety of segments of the desired patterns. A proton (H.sup.+) 12 on the substrate ide of epitaxial layer 11 is accelerated, and is radiated towards the epitaxial layer 11 which acts as a membrane so that some protons 12 are trapped between the monocrystal lattice 10 while selectively allowing an ion shower 13 to be emitted through grooves 15 in the form of the pattern. Ion shower 13 is emitted where grooves 15 occur because at grooves 15 silicon epitaxial layer 11 is relatively thin, thereby allowing ion beam exposure to occur.
Ion beam exposure mask 14 is formed by growing a silicon epitaxial layer 21, which is doped with boron, on one surface of a monocrystalline silicon substrate 20 to a thickness of about 2 .mu.m (FIG. 2a). An SiO.sub.2 film 22 and an Si.sub.3 N.sub.4 film 23 are formed on silicon epitaxial layer 21 by chemical vapor deposition. An Si.sub.3 N.sub.4 film 26 is then formed on the second or back surface of monocrystalline silicon substrate 20 (FIG. 2b). Si.sub.3 N.sub.4 film 26 is etched and the remaining portion serves as a mask for etching monocrystalline substrate 20 (FIG. 2c).
Anisotropy etching is performed on the entire substrate to remove a portion of monocrystalline silicon substrate 20, SiO.sub.2 film 22 and Si.sub.3 N.sub.4 films 23 and 26 (FIG. 2d). Monocrystalline silicon substrate 20 is then affixed to a pyrex glass ring 24 (FIG. 2e). A resist layer 25 is selectively formed on silicon epitaxial layer 21 by anisotropy etching (reactive ion etching; RIE or the like) forming a pattern of grooves 19 in silicon epitaxial layer 21 which removes resist layer 25 (FIG. 2g).
FIGS. 3 and 4 show an ion beam gun and ion beam exposure mask as disclosed in J. N. Randall, "Prospects For Printing Very Large Scale Integrated Circuits With Masked Ion Beam Lithography", J. Vac. Sci. Technol., A4 (3), May/June 1986, American Vacuum Society, pp. 777-783 are depicted. FIG. 3 shows the proximity exposure method of exposing the ion ray through the mask directly onto a wafer without distortion of size of the pattern using a parallel ion ray. An ion beam 30a emitted from an ion beam source 30 passes through an ion lens 31, a magnetic mass filter 32 and a deflection plate 33 through an exposure mask 34 onto a wafer 35. Ion beam 30a which passes through mask 34 produces the pattern of mask 34 on wafer 35.
FIG. 4 illustrates a method wherein ion beam 30a is radiated through a mask 34 almost at ion source 30 and is scaled down by an immersion lens 36 and passed through an octopole 37, a projecting lens 38 and an image reducer 39 adjacent to wafer 35. In this method the ion beam is masked into the pattern first and then passed through a series of beam changing devices prior to producing the pattern on the wafer.
These are the known prior art ion beam exposure devices and exposure methods currently in use. While these devices have been satisfactory, they suffer from the problem that the ion beam diameter of the ion beam gun varies with the diameter of the tungsten wire pin. Furthermore, the vacuum properties of the gun do not increase where a radio- active atmosphere is a vacuum. This is due to the fact that a liquified gas must be used in the tungsten wire pin method, and it is difficult to maintain vacuum properties equivalent to those obtained in the pin hole method.
It is possible to improve the parallel property of the ion beam with higher ion current values, yet it is difficult to obtain a large ion current. Thus, the ion current density is reduced during the developing of the point light source into a face light source. Therefore, the exposure time at the time of conducting the ion beam exposure methods becomes longer. This results in a decrease in productivity. Moreover, in the ion beam exposure mask there are problems because the thickness of the membrane cannot be easily controlled. Additionally, it is difficult to form an extremely thin membrane and if a thin membrane is formed it is difficult to handle because the membrane brakes easily. Finally, the prior art ion beam exposure masks cannot act as the ion source. Furthermore, in the proximity exposure method, the resolving power is low, thereby making it difficult to set the proximity gap. In the scale down projecting exposure method, the output is low.
Accordingly, it is desirable to provide an ion beam exposure device containing an ion beam gun and an ion mask which overcomes the shortcomings of the prior art devices described above.